1. Field of the Invention
The invention relates to a vehicle control system and, more particularly, to a vehicle control system that includes a rotary electric machine connected to a drive shaft of a vehicle via a transmission and an inverter that operates on a system voltage that is stepped up or down by a converter and that drives the rotary electric machine.
2. Description of the Related Art
A rotary electric machine mounted on a vehicle is operated by a circuit, such as an inverter, and, for example, drives a drive shaft of the vehicle via an adequate transmission. Thus, the rotational speed and torque of the rotary electric machine may possibly fluctuate depending on, for example, operation of the transmission or a load from a road surface, or the like, via the transmission.
For example, Japanese Patent Application Publication No. 2006-325374 (JP-A-2006-325374) describes that a step-up converter is provided between a direct-current power source and an inverter in an electric vehicle equipped with a transmission between an electric motor and a drive shaft, and also describes a load driving device for controlling a voltage output from the step-up converter on the basis of an output of the electric motor. The load driving device carries out torque reduction process to reduce the output torque of the electric motor during shifting in order to prevent an increase in rotational speed of the electric motor due to engagement and release of frictional elements at the time when the transmission shifts gears. Then, as a stepped-up voltage is reduced at that time, it causes unstable control because rectangular-wave control having a long control period assumes that the stepped-up voltage is constant. Thus, JP-A-2006-325374 describes that the stepped-up voltage is not reduced at the time when the transmission shifts gears.
Japanese Patent Application Publication No. 2007-20383 (JP-A-2007-20383) describes a controller for a vehicle that includes a motor that drives a wheel, an inverter that drives the motor, and a step-up converter that supplies electric current from a direct-current power source to the inverter. In the controller, for a torque constant command, as the wheel slips, the rotational speed of the motor steeply increases and, therefore, electric power consumption increases. Thus, control for supplying the motor with a larger amount of electric power is executed. However, when the wheel gets a grip thereafter, the rotational speed of the motor steeply decreases and, therefore, electric power supplied from the step-up converter becomes excessive. Then, JP-A-2007-20383 describes that a slip is detected on the basis of an increase in q-axis current to switch from rectangular-wave control over to PWM control and to reduce a target voltage of the step-up converter, and, in addition, the target voltage is reduced on the basis of a gear ratio if equipped with a transmission.
Japanese Patent Application Publication No. 2006-94691 (JP-2006-94691) describes a power output device. The power output device executes control, such as torque limiting, or the like, by calculating input/output allowable limits of a battery from a differential electric power obtained by subtracting assumed electric powers respectively input to MG1 and MG2 from an input/output electric power of the battery and Win and Wout of the battery when the driving state of an electric motor greatly changes because of a shift of a transmission, or the like. Then, JP-2006-94691 describes that, when the driving state of the electric motor greatly changes because of a shift of the transmission, or the like, the differential electric power also greatly changes, so the time constant used for calculating the differential electric power is reduced to prompt calculation of the differential electric power to thereby prompt process, such as torque limiting, thus preventing charging or discharging of the battery with an excessive electric power.
Japanese Patent Application Publication No. 2006-315451 (JP-A-2006-315451) describes a power output device that connects an engine, a first motor and a drive shaft to a planetary gear mechanism and that connects a second motor to the drive shaft via a transmission. In the power output device, as the voltage between battery terminals exceeds an upper or lower threshold, a switching state flag is set at 1, and, as the switching flag becomes 1 while the transmission is in upshifting, a hydraulic pressure command to a brake of the transmission is changed to decrease a rate of change in rotational speed of the second motor. JP-A-2006-315451 describes that this can prevent an excessive electric power input to or output from the battery due to a variation in electric power consumption of the second motor caused by a delay of sensing, or the like.
As described in the existing arts, the torque and rotational speed of the rotary electric machine connected to the transmission fluctuate depending on the operating state, or the like, of the vehicle. In this case, for example, as the electric power consumption of the rotary electric machine steeply varies because of a steep variation in rotational speed and torque of the rotary electric machine, a balance in electric power in a power supply circuit that drives the rotary electric machine greatly varies. This may, for example, cause an overvoltage or an undervoltage. In this case, an electrical component, such as a capacitor, that constitutes the power supply circuit for driving the rotary electric machine is subjected to a voltage above a rated value and may be damaged. Particularly, with miniaturization of electrical components, or the like, in order to provide a small light-weight vehicle-mounted unit, the rated values thereof also decrease. Therefore, it is necessary to take into consideration an overvoltage, or the like.